This invention generally relates to color picture tubes and, more particularly, to tension mask assemblies for cathode ray tubes capable of detensioning.
A color cathode ray tube, or CRT, includes an electron gun for forming and directing three electron beams to a screen of the tube. The screen is located on the inner surface of a faceplate panel of the tube and is made up of an array of elements of three different color-emitting phosphors. A shadow mask, which may be either a formed mask or a tension mask, is interposed between the electron gun and the screen. The electron beams emitted from the electron gun pass through apertures in the shadow mask and strike the screen causing the phosphors to emit light so that an image is displayed on the viewing surface of the faceplate panel.
A tension mask comprises a set of strands that are tensioned onto a mask frame to reduce their propensity to vibrate at large amplitudes under external excitation. Such vibrations would cause gross electron beam misregister on the screen and would result in objectionable image anomalies to the viewer of the CRT.
The mask stress required to achieve acceptable vibration performance is below the yield point of the mask material at tube operating temperature. However, at elevated tube processing temperatures, the mask""s material properties change and the elastic limit of the mask material is significantly reduced. In such a condition, the mask stress exceeds the elastic limit of the mask material and the material is inelastically stretched. When the tube is cooled after processing, the strands are longer than before processing and the mask frame is incapable of tensing the mask strands to the same level of tension as before processing. Another common problem is when the mask strand material has a lower coefficient of thermal expansion than the mask frame material. In such a case, tension on the mask strand increases during thermal processing, causing more inelastic strain.
It is, therefore, desirable to develop a mask frame assembly that allows tension masks to be uniformly detensioned during the thermal cycle used to manufacture a CRT to mitigate stretching of the mask.
The present invention relates to a detensioning mask frame assembly. The invention causes the mask frame to compress inward onto itself when heated during the CRT manufacturing process, thus relieving tension from the tension mask
More specifically, the detensioning mask frame assembly of the present invention comprises a dual-compliance system where opposite edges of a tension mask are attached to two parallel mask holding blades. The holding blades are attached to the centers of two opposite sides of a subframe. The subframe is constructed of two or more materials having different coefficients of thermal expansion whereby an increase in temperature causes the subframe to deflect causing the holding blades to move toward each other. The motion of the holding blades is essentially planar (XY-plane) allowing the holding blades to move without distortion of the tension mask contour. When the holding blades move toward each other, the strain in the tension mask is relieved and the stress is reduced. Hence, at elevated temperature, the subject invention detensions the tension mask without warping the tension mask contour.
This, in turn, reduces the inelastic strain experienced by the mask during thermal processing and retains more tension in the strands after thermal processing.